Fun. Had these once, exchanged them for Amphidinium then beat those. Upgraded my 120 to a 180 and even used 60% TB saltwater rock and 40% brand new thinking the flora / fauna would be beneficial for keeping them at bay. Threw out all the old rock / sand and FW dipped corals and got all new equipment hoping I wouldn't transfer. Guess it's back to the fight
Well if you beat Amphids, ostreos are a walk in the park. Love to hear your method against Amphids. Folks really struggle there sometimes. End up pulling the sand bed. And yeah, very common to have one replace another.
I would've hoped the live rock diversity would have kept you safe. Did you have any nutrient going when they appeared? How old is this set up?